The present invention relates generally to laser cutting probes for use in surgical procedures. More particularly, this invention pertains to a solid-state erbium laser surgical cutting probe and system for use in surgical procedures requiring high precision, including intraocular surgical procedures such as retinotomy, vitrectomy, retinectomy, capsulotomy, sclerostomy, and goniotomy.
Laser cutting, ablation and vaporization are common techniques in surgery. For example, the CO.sub.2 laser has been used in dermatology for cutting tumors. Corneal cutting and reshaping is being performed with excimer photorefractive surgery (PRK). The best laser wavelength for many procedures is the wavelength having the highest absorption in tissue. Because water comprises the highest component of the tissue, the best water absorption wavelength often has the best cutting effect.
The most commonly used lasers for cutting are CO.sub.2 laser at 10.6 .mu.m (water absorption coefficient=8.5.times.10.sup.2 /cm.sup.-1), and Er:YAG at 2.94 .mu.m, (water absorption coefficient=1.3.times.10.sup.4 /cm.sup.-1).
Intraocular cutting techniques are needed for ophthalmic surgery. Laser intraocular cutting may improve the surgery with smaller incisions, easier control, and higher precision. Several studies have tried different lasers and various delivery devices to develop this technique.
Lewis A. et al. in the Hebrew University of Jerusalem, Israel, guided an excimer laser beam (193 nm) with an articulated mechanical arm and confined it with a variable-diameter tapered tube (1 mm to 125 .mu.m in diameter). An air stream was used to push the intraocular liquid out of a cannula and remove fluid from the retina surface just in front of the needle tip. With such an excimer laser delivery system, it was possible to remove retinal tissue accurately without collateral damage.
Dodick J M, et al tried to overcome the delivery problems by conducting a short pulse Nd:YAG laser (1.064 .mu.m) into the eye with a silica fiber. At the end of the surgical probe, the short pulses hit a titanium target and generated shock waves. This device was applied to fragment nuclear material for cataract extraction.
D'Amico et al delivered Er:YAG laser through a fluoride glass fiber to an endoprobe with sapphire or silica fiber tips ranging from 75 to 375 .mu.m. This probe was used for transection of vitreous membranes, retinotomy, and incision and ablation of epiretinal membranes. Results showed that twenty-five vitreous membrane transections were made in 16 eyes at distances ranging from 0.5 to 4.5 mm from the retina with radiant exposures ranging from 2 to 50 J/cm2 (0.3-5.5 mJ) with nonhemorrhagic retinal damage in a single transection. Sharp, linear retinotomies were created successfully in five eyes. Epiretinal membrane ablations were performed with radiant exposures ranging from 1.8 to 22.6 J/cm2 (0.3-2 mJ). In aqueous media, results of microscopic examination showed partial- to full -thickness ablation with a maximum lateral thermal damage of 50 .mu.m. In air- and perfluoro-N-octane-filled eyes, there was increased lateral damage with desiccation of residual tissue. In 12 aqueous-filled eyes, 18 linear incisions were successfully performed, with retinal nonhemorrhagic damage in 2 eyes and hemorrhage in 5. Based on these results, a commercial florid glass delivered Er:YAG laser has been developed for further research (VersaPulse.RTM.Select.TM. Erbium, Coherent.RTM., Palo Alto, Calif.).
Joos K, et al delivered Er:YAG laser (2.94 .mu.m) through ZrF fiber and coupled to a short piece of sapphire fiber (Saphikon, Inc., Milford, N.H.), or low-OH silica fiber at the end of the intraocular probe. The probe was combined with an endoscope to perform goniotomy in vitro and in vivo. The results showed that minimum tissue damage created with the Er:YAG energy was at an energy level of 2 to 5 mJ per pulse.
Pulsed Er:YAG laser at 2.94 .mu.m wavelength is capable of cutting human tissue with high precision and little thermal damage to the surrounding tissue. The potential applications include photo-refractive keratectomy, plastic surgery, and intraocular cutting surgery such as retinotomy, vitrectomy, capsulotomy, goniotomy, etc.
To understand the proper use operation of a micro-Er:YAG laser in the surgical applications, it is important to understand its dynamics. The energy level scheme of a 970 nm diode pumped Er.sup.3+ in a YAG crystal is shown in FIG. 4. The 970 nm diode directly pumps the Er.sup.3+ to the upper laser energy level. The laser action occurs between the .sup.4 I.sub.11/2 -.sup.4 I.sub.13/2 states. Each of these states is Stark split into about 6 to 7 branch energy levels by the crystal field. These levels are thermally populated as described by a Boltzmann distribution. When the X2 branch of .sup.4 I.sub.11/2 is relatively higher populated than the Y7 branch of .sup.4 I.sub.13/2, the 2.94 .mu.m laser transition will occur, even when the entire .sup.4 I.sub.11/2 and .sup.4 I.sub.13/2 levels are not inversed. Continuous wave (CW) and quasi-cw laser operations of this transition at room-temperature have been reported with high efficiency and high output power. When the population of .sup.4 I.sub.13/2 accumulates to a certain density, two neighboring Er.sup.3+ ions can interact. One ion jumps to the higher energy .sup.4 I.sub.9/2 and the other one jumps to the lower level .sup.4 I.sub.15/2. Then the .sup.4 I.sub.9/2 level ion will relax to the upper laser level .sup.4 I.sub.11/2 by rapid multi-photon transition within about 1 .mu.s. This is called the up-conversion process. It is responsible for shortening the lifetime at the lower laser level which simultaneously leads to excitation of the upper laser level.
Since lifetime at the .sup.4 I.sub.11/2 level is shorter than at the .sup.4 I.sub.13/2 level, highly doped crystals such as YAG:Er.sup.3+ at 50% concentration have to be used to reduce the lifetime at the lower laser levels, and increase the probability of interaction at the lower level laser ions
All commercial Er:YAG lasers presently are flash light pumped. Some of the companies which produce the Er:YAG lasers are: Big Sky, SEO, LSD, Kigre, FOTONA, Quantex, etc. The most common feature of these lasers are: wavelength--2.94 .mu.m; pulse length--150 to 300 .mu.s; energy per pulse--100 to 1000 mJ; and repetition rate--1 to 20 Hz.
Other manufacturers (e.g. Coherent, Premier, and Candela) produce fluoride glass delivered Er:YAG laser. Because all fluoride glass fibers can not withstand high laser power, these lasers normally have outputs of less than 20 mJ per pulse of energy.
Kigre Inc. produces a small Er:YAG laser, which places a small flash light pumped Er:YAG laser head into a pistol style hand piece. However, it requires high voltage power for the flash light and the hand piece is not small enough for intraocular surgery.
There is no commercialized diode pumped Er:YAG laser. Theoretical and preliminary experiments showed that a diode pumped Er:YAG laser has a much higher conversion efficiency (10% to 20%) than flash light pumped ones (efficiency &lt;2%). Dinerman, et. al. used a 970 nm diode laser and a Ti:sapphire laser to end pump a 3 mm long Er:YAG laser. This produced 143 mW of cw power when the pump power was 718 mW. Hamilton, et. al. pumped a 2.times.2.times.14 mm Er:YAG laser rod with a pulsed diode laser array bar with 200 W peak power, and reached the maximum output energy of 7.1 mJ per pulse at 100 Hz repetition rate. The parameters and results of these experiments are listed in Table I:
TABLE 1 Previous experiments of diode pumped Er:YAG lasers Er:YAG #1 Er:YAG #2 Er:YAG #3 Pump wavelength: 970 nm 963 nm 963 nm source Max. peak 718 mW 200 W 200 W para- power: 88 mJ 88 mJ meters: Max. pulse cw 400 .mu.s 400 .mu.s energy: pulse length: Er:YAG rod size: .PHI.3 .times. 3 mm 2 .times. 2 .times. 14 mm 1 .times. 1 .times. 14 mm laser Er.sup.3+ 33% 50% 50% para- concentration: 3 mm 40 mm 25 mm meters: cavity length: 99.7% 98% 98% output coupler: Output pump 410 mW results: threshold: total efficiency: 8% 5.3% slope 12% 13% efficiency: Max. peak 171 mW 17 W 11.75 W power: 7.1 mJ 47 mJ Max. pulse cw 100 Hz 275 Hz energy: repetition rate:
These results indicate that using a diode array pumped micro-Er:YAG laser is very promising for low energy requirements.
One of the biggest difficulties in the practical application of the erbium laser for such surgical applications is that none of the available fibers are ideal for the delivery of the laser. In addition to the bulky mechanical articulated arm, fiber optics available for these wavelengths are: fluoride glass fiber, single-crystal sapphire fiber, chalcogenide glass fiber, polycrystalline fiber, and hollow waveguides. Although they offer some ability to deliver different cutting wavelength, all of them have one or more defects of: brittleness, water solubility, toxicity, sensitive to UV exposure, limited mechanical strength, low temperature damage threshold, and low laser damage threshold. The articulated arms and sapphire fibers are not flexible enough for effective use by the surgeon. Although fluoride glass fibers have better flexibility, they are too brittle and their damage energy thresholds are too low for intraocular use.
What is needed, then, is a cutting probe system that is sufficiently flexible and mechanically strong to reliably deliver the energy from an erbium laser for use in intraocular surgery and in other surgical procedures requiring high precision.